Recent international regulations governing the power quality and harmonic currents pollution of utility power lines has placed an increased emphasis on the problem of interfacing electronic DC loads to the AC utility power line with an ac-to-dc converter. The conventional way of doing this ac-to-dc conversion is to use a full-wave bridge rectifier and a capacitor-input filter followed by a voltage regulator (dc-to-dc converter).
The main problem associated with a capacitor-input filter is that narrow-pulse, high peak currents result which produce high harmonic currents on the utility line. These large harmonic currents are undesirable because they produce distortion of the line voltage and conducted and radiated electromagnetic interference (EMI). Only the components of input current which are of the same frequency and in phase with the input voltage deliver active power to the load. For ideal sine-wave line voltage, higher order harmonic currents do not contribute to load power but only generate the increased rms currents in the transmission lines and, therefore, produce additional losses. When input current is made proportional to the line voltage, maximum active power is delivered and ideal unity power factor is achieved.
The increasing number of electronic DC loads that are being connected to an AC power line and their sensitivity to the quality of the line voltage require from the designer and user of an AC power line consideration of not only the DC voltage provided but also the quality of the input current. Thus, the following objectives are presented which may be closely related but not identical: achieving unity power factor; and minimizing harmonic current content. In the special case of an ideal sine-wave line voltage, these two objectives are identical. However, the line voltage contamination with harmonic currents is the more severe problem which will be regulated by international regulations, such as IEC 555-2 coming into effect in December 1994.
Close voltage regulation is essential when an ac-to-dc converter is to supply DC voltage to sensitive loads. In applications requiring a converter with multioutput voltages, or in distributed power supply systems, a good solution is to have a front-end input current shaper with a single isolated output (DC bus voltage) and one or more down-stream post-regulators (dc-to-dc converters) to provide close regulation on each DC output voltage. This approach has several advantages: isolation and safety requirements are satisfied in the current shaper; and the regulation of the DC bus voltage does not have to be very tight nor have fast response since the post-regulators are present.
The process of improving the input-current waveform of an ac-to-dc converter with the goal of alleviating problems of noise and line-voltage distortion and improving the power factor is called input current shaping, and a device which can perform input current shaping is referred to hereinafter as a current shaper. For a maximum power delivery and therefore unity input power factor, every current component drawn from the line must be related to its corresponding voltage component by a common scalar. So, if the line voltage is distorted, the input current also needs to be distorted. Therefore, the current shaper needs to behave like a resistor, i.e., needs to emulate a resistor.
The functions of the current shaper are: to shape the input current; to "balance" the difference between input and output power; to provide output voltage regulation; and optionally to provide isolation between the line and the load.
The use of active methods for input current shaping based on a switching dc-to-dc converter is the best way to achieve high input power factor. There are numerous papers describing application of various dc-to-dc converters in current shaping applications. [S. D. Freeland, "Input Current Shaping for Single-Phase Ac-Dc Power Converters," Ph.D. Thesis, Part II, California Institute of Technology, 1988; L. H. Dixon, Jr., "High Power Factor Pre-regulators for off-line Power Supplies," Unitrode Power Supply Design Seminar SEM-800, 1991, pp. I2-1-I2-16; M. J. Kocher and R. L. Steigerwals, "An Ac to Dc Converter with High Quality Input Waveforms," IEEE Power Electronics Specialists Conference, 1982 Record, pp. 63-75 (IEEE Publication 82CHI762-4); and J. Sebastian, J. Uceda, J. A. Cobos, J. Arau, F. Aldana, "Improving Power Factor Correction in Distributed Power Supply Systems Using PWM and ZCS-QR Sepic Topologies," IEEE Power Electronics Specialists Conference, 1991 Record, pp. 780-791 (IEEE Publication 91CH3008-0). All of the converters usually operate in continuous conduction mode (CCM), and control circuits widely used are based on programming input current to be proportional to the line voltage by closing an input current feedback loop.
Using dc-to-dc converters in discontinuous inductor current mode (DICM) of operation for input current shaping applications is very attractive for low power levels because very simple control circuits can be used [D. Chambers and D. Wang, "Dynamic P. F. Correction in Capacitor Input Off-Line Converters," Proc. Sixth National Solid-State Power Conference (POWERCON 6), Miami Beach, Fla., May 2-4, 1979, pp. B3.1-6]. When a converter operates at constant switching frequency and constant duty cycle current, shaping is obtained automatically.
A boost converter operating in DICM is very often used as a current shaper for low power applications where isolation between line and load is not required. When it operates as an "automatic" current shaper, the input current is not a linear function of the line voltage, and an input power factor greater than 0.97 is theoretically possible for a conversion ratio M&gt;1.5 [S. D. Freeland, supra]. An alternative method is to operate at the boundary between continuous and discontinuous inductor current mode (CICM and DICM), but it requires a variable switching frequency and a complex control circuit with a multiplier. [B. Andreycak, "Controlled ON-Time, Zero Current Switched Power Factor Correction Technique," Unitrode Switching Regulated Power Supply Design Seminar Manual, SEM-800, 1991, pp. 3.1-3.10]. The disadvantage of this control method is that switching frequency varies over a wide range with load and line voltage which in many applications is unacceptable. Although a current shaper using a boost converter is very simple and popular, it has some very serious practical drawbacks, such as:
Output voltage is always higher than the peak input voltage; PA1 Isolation cannot be easily implemented; PA1 High in-rush current exists during start-up; PA1 There is no overload protection because there is no active switch between input and output.
Also, the output capacitor will attempt to charge resonantly to twice the input voltage during start-up which cannot be tolerated in distributed power supply systems. To solve these problems effectively, an additional active switch and its control circuit are needed.
A flyback converter is very popular when operating in DICM since the average input current exactly follows the input voltage if switching frequency and duty ratio are kept constant. [S. D. Freeland, supra; D. Chambers, et al., supra; and R. Ericson, M. Madigan and S. Singer, "Design of a Simple High-Power-Factor Rectifier Based on the Flyback Converter," IEEE Applied Power Electronics Conference, 1990 Record, pp. 792-801]. Sepic converters have also been proposed as automatic current shapers operating in DICM [J. Sebastian, et al., supra]. Both types of converters overcome drawbacks associated with a dc-to-dc converter used as a current shaper for low power applications.
When a dc-to-dc converter operates in DICM, the high switching current ripple is present with a magnitude at least twice the average value of the input current for variable switching frequency, or even higher if the switching frequency is kept constant. High switching frequency and additional filtering are required to reduce harmonic distortion, and to get a high input power factor. Neither of these approaches are optimal since they reduce overall efficiency of the current shaper and increase size, weight and number of magnetic components.